Interaction of biomass ash and bed materials in thermochemical conversion in fluidized beds leads to changes of the bed particle surface due to ash layer formation. Ash components present on the bed particle surface strongly depend on the ash composition of the fuel. Thus, the residual biomass used has a strong influence on the surface changes on bed particles in fluidized bed conversion processes and, therefore, on the catalytic performance of the bed material layers. Ash layer formation is associated with an increase in the catalytic activity of the bed particles in gasification and plays a key role in the operability of different biomass fuels. The catalytic activation over time was observed for K-feldspar used as the bed material with bark, chicken manure, and a mixture of bark and chicken manure as fuels. The changes on the bed material surfaces were further characterized by SEM/ EDS and BET analyses. Raman, XPS, and XRD analyses were used to characterize the crystal phases on the bed material surface. An increase in surface area over time was observed for K-feldspar during the interaction with biomass ash. Additionally, a more inhomogeneous surface composition for fuels containing chicken manure in comparison to pure bark was observed. This was due to the active participation of phosphorus from the fuel ash in the ash transformation reactions leading to their presence on the particle surface. A decreased catalytic activity was observed for the same BET surface area compared to bark combustion, caused by the different fuel ash composition of chicken manure.

The reaction kinetics for carbonization of biogas slurry and the influence of diffusion for high-density feedstocks (pistachio shell) were investigated. The experiments were carried out with a 1.5 L batch reactor for hydrothermal carbonization at reaction temperatures of 190, 200, and 220 °C and a residence time of 0–160 min. The obtained hydrochar samples were dried and analyzed via scanning electron microscopy and a C, H, and O elemental analyzer. It was found that the average activation energy of the biogas slurry is 50.7 kJ/mol. In the case of the diffusion experiments, Thiele moduli of 1.21–1.61 were found, which indicates an influence on diffusion for the hydrothermal carbonization of pistachio shells.

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